The present invention relates to a sewing machine, and more particularly, a bobbin for a sewing machine, in which an under thread is stably pulled out without entanglement or cut of the under thread.
Generally, a sewing machine, as shown in FIG. 1, has a rotary shuttle device for supplying an under thread. The rotary shuttle device comprises an outer shuttle which is disposed at a driving shaft 1 rotatably connected to a power generating unit, an inner shuttle 3 which is received in the outer shuttle 2, a bobbin 4 which is received in the inner shuttle 3 and on which the under thread is wound, and a bobbin case 5 for receiving the bobbin 4. The bobbin 4 is comprised with a cylindrical winding shaft 4a on which the under thread is wound, and a flange 4b which is fixed to both end of the winding shaft 4a to restrict a winding amount of the under thread.
However, a maximum amount or a maximum thickness of the under thread substantially wound on the bobbin 4 is set to a maximum amount that the under thread wound around a descending point of a sewing needle is not contacted with the sewing needle during a sewing operation. In a convention bobbin, an amount of the under thread wound on the bobbin is very much restricted due to a reason set forth above. Therefore, since a user or an operator has to frequently replace an empty bobbin with a new one, there is a problem that the efficiency in a sewing operation is lowered.
Recently, in order to solve the problem of the conventional bobbin, i.e., be capable of winding a much more amount of the under thread on the bobbin, as shown in FIG. 2, there has been proposed a bobbin 6 in which a winding region of the under thread is divided into two portions. The bobbin 6 having two winding regions is provided with a first flange 6b having a relatively large diameter which is formed at one end of a winding shaft 6a, and a second flange 6c having a relatively small diameter which is formed at the other end of the winding shaft 6a. Particularly, at a center portion of the winding shaft 6a, there is formed a partition flange 7. A trough portion 7a and a ridge portion 7b are alternately formed at an outer circumference of the partition flange 7 so that the under thread facilely passes over the partition flange 7. Between the first flange 6b and the partition flange 7, there is formed a first winding portion 8a on which a relatively large amount of the under thread is wound. Between the second flange 6c and the partition flange 7, there is formed a second winding portion 8b on which a relatively small amount of the under thread is wound.
As described above, a reason why the amount of the under thread wound on each winding portion 8a, 8b is different is that the wound amount of the under thread is restricted by the a plurality of trough portions 7a and ridge portions 7b formed at the partition flange 7 as well as a width or height of each flange 6b, 6c formed at both ends of the winding shaft 6a. 
In the bobbin having the partition flange at the center portion thereof, the wound amount of the under thread is somewhat increased. However, there are also some problems. The problems will be described below.
FIG. 2b is a perspective view showing a state that an under thread is pulled out of the conventional bobbin of FIG. 2a. An xe2x80x9cAxe2x80x9d designates the under thread passed over from the second winding portion 8b to the first winding portion 8a. A portion, over which the under thread is passed, is correspondent to a boundary portion between the rough portion 7a and the ridge portion 7b. However, it is not consistent. In this situation, before the under thread wound on one winding region is completely pulled out and the winding region is then changed to the other winding region beyond the partition flange during the sewing operation, the under thread in the state of xe2x80x9cAxe2x80x9d may be previously slipped off from the trough portion or the ridge portion of the partition flange in an arrow direction due to rotation of the bobbin and then changed in a state of xe2x80x9cBxe2x80x9d. If the under thread is previously slipped off, the slipped under thread is not deviated from the bobbin for a time period as long as a length of the slipped under thread and thus entangled with other under thread which is continuously pulled out. As the result, the under thread is not normally supplied to the sewing region. The under thread will be cut by such phenomenon in the end. Accordingly, the sewing operation is stopped, thereby lowering workability and also reducing the quality of a sewn product.
Therefore, it is an object of the present invention to provide a bobbin for a sewing machine, which is capable of preventing an under thread from being entangled or cut when being pulled out of the bobbin.
It is other object of the present invention to provide a bobbin for a sewing machine, which is capable of preventing an under thread from being previously slipped off when the under thread passes over a partition flange between a first winding portion and a second winding portion.
To accomplish the above objects and advantages, there is provided a bobbin for a sewing machine comprising a winding shaft which is separatably inserted onto a fixing shaft of the rotary shuttle device and on which the under thread is substantially wound, a first flange which is integrally formed at one end of the winding shaft, a second flange which is integrally formed at the other end of the winding shaft and a partition flange for forming a first winding portion between the first flange and the partition flange and a second winding portion between the second flange and the partition flange, characterized in that a plurality of trough portions and ridge portions are alternately formed at an entire outer circumference of the partition flange, each of the trough portions and the ridge portions has the same gradient each other, and a side portion is formed at both sides of each trough portion to prevent the under thread from being previously slipped.